BIOS

From Sega Retro

The Basic Input-Output System or BIOS is computer interface code that locates and loads the operating system into RAM. It provides low-level communication, operation and configuration to the hardware of a system, which at a minimum drives the joypad and provides primitive output to a display. The BIOS is usually written in the Assembly language native to the processor. Listed below is information regarding the attraction modes to Sega's hardware systems, which are the sequences displayed to the end user upon starting up the system.

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Sega Master System

Based on the Sega Mark III in Japan, the Sega Master System (SMS) had a BIOS just like any other computer interface. Almost all of Sega's hardware systems had some form startup sequence that graphically told the end user that a game was loaded and ready to play. However, what was unique about SMS was that Sega had included built-in games that could be played without any cartridges inserted.

The first known BIOS released was based upon the Sega Mark III, which featured an animated blue SEGA logo, along with MASTER SYSTEM written underneath. This was a prototype that wasn't released in the final product and is designated as Prototype M404 among emulation communities. (See here for video demonstration)

Among its final release in North America in June 1986, the first major BIOS known publicly is the SEGA startup sequence. The animation sequence this time was the SEGA logo sliding from the left side in the center of the screen, followed by MASTER SYSTEM and its copyright information shown underneath. The trademark SEGA sound as heard in many of the Sonic the Hedgehog games, is also played in the background using melodic system tones (See here). Although less animated, there was an extra hidden game that could played upon startup. By pressing 1, 2 and UP on the controller, the player could play the infamous Snail game.

Japanese releases of the SMS BIOS offered more musical graphical enhancements, along with music from Space Harrier (See here).

Finally, Sega also packaged other games with the system built-in similar to the Snail game, but was no longer hidden to the end user. Depending on the console build and BIOS included with the system, players could choose to play either Hang On, Missile Defense 3-D, and Safari Hunt.

Sega Mega Drive / Genesis

Early production runs of the Sega Mega Drive did not include a boot ROM; instead, they instantly started the ROM cartridge if one was present, or booted to a device on the expansion port if a cartridge was not present. In 1990, Sega introduced the TradeMark Security System (TMSS), which includes a 2 KB startup ROM that runs before the ROM cartridge if a cartridge is present. The TMSS program checks the ROM cartridge to make sure it has the correct copyright information. If it does not, the system halts. Otherwise, it displays a message saying "PRODUCED BY OR UNDER LICENSE FROM SEGA ENTERPRISES LTD.", pauses for a few seconds, and then continues to load the game.

Once the game is loaded, the TMSS ROM is mapped out of the system address space, making it inaccessible to the game program.

Sega Mega-CD

The Sega Mega-CD, known as the Sega CD in North American audiences, featured a Boot ROM similar to that of the Sega Master System. The Boot ROM provided an Audio CD player interface as well as CD-ROM access functionality utilized by Sega CD games. Multiple versions of the Boot ROM were released for different regions and hardware revisions.

Since the Sega CD Boot ROM contains the CD-ROM access functions, Sega was able to change parts of the CD-ROM hardware without breaking existing games. However, this does mean that a Boot ROM designed for e.g. Model 2 consoles may not necessarily work properly on Model 1 consoles and vice-versa. Sega CD emulators are typically designed around Model 1 hardware, and hence may have problems with Boot ROMs designed for other systems, including Model 2, Wondermega, X'Eye, and CDX/MultiMega.

First and foremost, the Mega CD was not a standalone system. It was actually attached to any Model 1 or 2 Sega Mega Drive, giving the Mega Drive full control of the CD system. The standalone Multi-Mega and CDX systems, although combined as one machine, follow the same startup procedures as follows. If a cartridge based game is inserted into the MD, then the MCD remains idle. However, if there is no cartridge inserted, then the MCD gets full control and begins its sequences. If no CD is inserted, the attract music and SEGA animations continue to play until the player makes a decision. He or she can either continue watching the sequence, insert a game disc, or press A, B, or C to enter the CD-Player mode. From the CD Player, the user can either play music, manage items on the backup RAM, or again, insert a game disc.

Below are revisions of each BIOS version according to region and model. Clicking on the region will link you to a video demonstration of its attract mode. For CRC information, downloading of BIOS files, and further machines that feature the BIOS below, please see the BIOS section from the Sega Mega-CD page.